An interferometer is an instrument in which a beam of coherent electromagnetic radiation, e.g., optical radiation, is divided into signal and reference beams. The signal and reference beams are caused to travel different paths, and are then combined to produce an interference pattern. Measurement of the interference pattern can provide very sensitive information concerning the lengths of the paths traversed by the respective beams. If the signal beam is reflected off a test structure being monitored for thermal distortion, the result is that very small thermally induced distortions can be detected. Alternatively, if the signal and reference beams are reflected off different portions of a test material, the coefficient of thermal expansion of the test material can be determined. In a multiple channel interferometer, the signal beam is divided into a plurality of signal beams, and each signal beam is reflected off a different point of a structure or material under test.
One early multiple channel interferometer for use in thermal distortion measurements is described in U.S. Pat. No. 4,105,336. In that interferometer, a laser beam is divided by a beam splitting device into signal and reference beams. The signal beam is spatially filtered by reflection off a small signal beam mirror, collimated, and passed through a window into an environmental chamber containing the test structure. The signal beam is reflected from four reflectors fastened to the test structure at spaced-apart positions, to produce a corresponding set of four reflected signal beams. The reference beam produced by the beam splitter is transmitted directly into the test chamber and is reflected from a reflector at the far end of the test chamber from the entrance window. This reflector produces a reflected reference beam that is slightly offset from the incoming reference beam. The reflected reference beam is spatially filtered by reflection off a small reference beam mirror, and then combined with each of the reflected signal beams to produce four composite beams. Both the signal and reference beam mirrors are attached to a common, mechanically vibrated structure, so that both beams are modulated at an identical frequency. The four composite beams are directed to four photodetectors, and the output of each photodetector is monitored to produce information concerning movement of the test structure. The modulation of the signal and reference beams by the vibrating signal and reference mirrors permits the direction of movement as well as the movement distance to be determined. This interferometer was used to measure the thermal distortion of the truss for the large space telescope, the truss consisting of cylindrical structure 6.5 meters long and 3.4 meters in diameter that was designed to hold the primary and secondary mirrors of a 3 meter telescope in alignment.
In order to improve the capabilities of the interferometer described above, a 19-beam interferometer was subsequently built that was capable of the simultaneous measurement of the relative movement of 19 positions on a single test structure. In this improved interferometer, acousto-optic modulators are used in place of vibrating mirrors to modulate the signal and reference beams at 80 MHz and 81 MHz respectively, to produce a 1 MHz different frequency that is used as a reference signal. The advantage of a 19-beam interferometer is that it can measure the complete relative movement of two positions on a test structure. In a typical setup, nine beams are used for each measurement position. Six beams are required for measuring 6 degrees of freedom of the test structure, and three beams are required for measuring the tilt of the nine-beam cluster relative to a reference plate. Such a 19-beam interferometer is described in U.S. Pat. No. 4,329,059. Although in principle, the interferometer described in that patent could be increased in size to permit the simultaneous measurement of more than two positions on a test structure, a practical limit to such an expansion is soon reached. For example, in order to measure the complete relative movement of ten positions on a test structure, 90 beams would be required, each beam requiring its own means for directing the beam along an appropriate optical path. The impracticality of such an instrument has to date prevented the expansion of the capabilities of multibeam interferometers described above.